Centrifugal liquid pumps are used in many environments or applications. Such devices conventionally include a housing defining a pump chamber or cavity within which an impeller assembly is rotated. The impeller assembly is mounted on a shaft rotatably journalled within the housing and including radially projecting impeller blades for drawing fluid into an inlet of the housing and discharging through an outlet. Bearings are provided about the impeller shaft, usually behind the impeller, to journal the shaft within the housing. A pressure balance chamber may be provided behind the impeller to reduce axial thrust loads, thereby increasing the life or reducing the size or quantity of the thrust bearings. Such pump systems commonly are of a seal type design and may employ single, tandem or double seal systems along a shaft as is deemed desirable in particular applications. These sealed pumps attempt, as much as possible, to contain the process fluid and to prevent fluid entry into the gear box or other driving components in gear driven high speed units.
However, sealless pumps, that is, pumps without shaft seals, are being employed in increasing numbers due to environmental concerns and, in some instances, in response to mandated legislation. These pumps typically are considered as being desirable when used with fluids which are hazardous, polluting or expensive. Generally, sealless pumps are more expensive and less efficient than the more common pumps described above, but these disadvantages are considered the price to be paid for a clean environment.
Generally, mainline electric driven sealless pump technology can be broken down into two concepts, namely a canned electric motor driven pump and a magnetic coupling driven pump. An inherent limitation of the canned motor pump concerns the skin friction loss of the submerged drive motor which imposes a viable speed limit, i.e. a head capability limit. Skin friction loss is the drag or resistance of the process fluid on the motor rotor. Series staging or series arranged canned motor pump components constitute complex and costly means of circumventing this limitation and are infrequently utilized. Conversely, modern design utilizing potent rare earth permanent magnets incurs relatively low skin friction loss in magnetic drives, thereby allowing practical high speed operation, i.e. high head design. Concentric arrangements of magnets generally are used, particularly for high torque applications, i.e. greater than one horsepower.
Permanent magnet driven pumps may be of the more common concentric magnetic geometry or may be of a less common axially facing magnetic geometry. Concentric magnetic arrangements have the advantage of producing only torque forces while the axial geometry imposes axially attractive forces on the drive and driven shafts, in addition to the prime objective of drive torque. The need to handle substantial axial thrust could impose serious disadvantages in pump design, particularly in the driven half of the magnetic coupling where the process fluid must serve as a lubricant. However, this disadvantage can be compensated for if the axial magnetic geometry is used in conjunction with a hydraulic thrust balance system where available hydraulic force easily accommodates the magnetic attraction force. Such thrust balance systems are shown in U.S. Pat. Nos. 4,867,633 to Gravelle, dated Sep. 19, 1989, and 5,061,151 to Steiger, dated Oct. 29, 1991, both of which are assigned to Sundstrand Corporation.
This invention is directed to various improvements in a sealless centrifugal pump of the impeller-type which employs an axial permanent magnet geometry in conjunction with a hydraulic thrust balance system.